The present invention relates to a method for the preparation of an article comprising a substrate containing silicon and a barium-strontium aluminosilicate barrier layer which functions as a protective environmental/thermal barrier coating.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures in aqueous environments. By aqueous environment is meant a water and/or steam environment. It has been found that these silicon containing substrates can recede and lose mass when exposed to high temperature, aqueous environments. For example, silicon carbide when exposed to a lean fuel environment of approximately 1 ATM pressure of water vapor at 1200.degree. C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH).sub.x. It has been found that barium-strontium aluminosilicate barrier layer as a top layer is effective to reduce formation of volatile silicon species and thus reduce recession and mass loss. However, the barrier layer has been found to exhibit cracks in the fabricated matrix. Naturally it would be highly desirable to provide an external barrier coating for silicon containing substrates which is substantially crack free (cracks through the barrier layer to the substrate) and which would inhibit the formation of volatile silicon species, Si(OH).sub.x and SiO, and thereby reduce recession and mass loss.
Accordingly, it is the principle object of the present invention to provide method for the preparation of an article comprising a silicon containing substrate with a barium-strontium aluminosilicate (BSAS) barrier layer which is substantially crack free and which inhibits the formation of gaseous species of Si, particularly Si(OH).sub.x, when the article is exposed to a high temperature, aqueous environment.
It is a further object of the present invention to provide a method for producing an article as aforesaid.